1. Field of the Invention
The present invention relates to a vehicle power converter that is connected between an AC generator-motor and a DC power supply.
2. Description of the Related Art
A vehicle power converter for performing power conversion between DC power and AC power is used for a vehicle generator-motor which is mounted in a vehicle and driven to start up an engine and generates electric power after the engine is driven.
In general, such a vehicle power converter is connected between a power generator-motor and a battery, and it comprises a plurality of switching elements and diodes elements connected to the respective switching elements in parallel (for example, parasitic diodes appended to the switching elements). When the power generator-motor operates as an electric motor, DC power from the battery is converted to AC power by controlling ON/OFF of the switching elements. On the other hand, when the power generator-motor operates as a power generator, AC power generated by the power generator-motor is rectified by the diodes to be converted to DC power.
However, synchronous rectification based on switching elements has been recently used in place of diode rectification because it has higher efficiency and heat generation of elements is less. According to the synchronous rectification, in synchronization with a conduction state of respective diodes, the switching elements corresponding to the diodes concerned are conducted to each other. For example, a vehicle power converter disclosed in JP-A-2008-228450 performs a synchronous rectification operation of controlling ON/OFF of switching elements in synchronization with the timing at which the diodes are energized.
Normally, this type of control system controls the power generator-motor so that when power generation current generated by the power generator-motor is small, the diode rectification is carried out by the diodes connected to the switching elements in parallel, and then when the power generation current thus generated increases, the diode rectification is shifted to the synchronous rectification.
JP-A-2008-228450 describes means for implementing the synchronous rectification in detail, but has no disclosure about a shift condition for the shift from the diode rectification to the synchronous rectification and the shift from the synchronous rectification to the diode rectification. Therefore, the method of JP-A-2008-228450 has the following problem. That is, when the rectification style is shifted from the diode rectification to the synchronous rectification under a low-rotation and low-load state, the induced voltage is lowered due to execution of the synchronous rectification, and thus on-timing and off-timing of diodes for comparing the induced voltage and a voltage Vp at a positive side terminal P for generation cannot be accurately calculated, and thus the synchronous rectification may fail.
As the voltage value of a power generation voltage is higher, a larger power generation current (load current) is required to shift the rectification style from the diode rectification to the synchronous rectification. Therefore, there is a problem that an operation area in which the synchronous rectification can be performed is narrow when the power generation voltage is high.